Semiconductor elements such as transistor, IC and LSI have heretofore been encapsulated with ceramic package or the like to obtain semiconductor devices. The recent trend is for more semiconductor elements to be encapsulated with a plastic package from the standpoint of cost and mass productivity. For such a resin encapsulating, epoxy resin compositions have heretofore been used with good results. However, the technical innovation in the field of semiconductors has brought about an increase in the size of elements and a reduction in the diameter of lead wires along with the enhancement of the degree of integration, showing a trend for smaller and thinner packages. With this trend, a higher reliability has been desired for encapsulating materials.
On the other hand, electronic components such as semiconductor device need to conform to UL94-V0, which is a flame retardance specification. Heretofore, it has been a common practice to incorporate a brominated epoxy resin and antimony oxide in an epoxy resin composition for encapsulating semiconductors in order to provide the epoxy resin composition with a flame retardance.
However, the above-described flame-retarding technique has two main problems.
The first problem includes environmental hazards such as the toxicity of antimony trioxide itself, the harmfulness of toxic gases generated during combustion, e.g., hydrogen bromide, bromic gas and brominated antimony, to human body, the corrosion of apparatus by these toxic gases, and the difficulty in disposal of industrial wastes discharged at the semiconductor element encapsulating step and used semiconductor devices.
Referring to the second problem, when a semiconductor device employing the above-described flame-retarding technique is allowed to stand at high temperatures for a prolonged period of time, it releases bromine which then corrodes aluminum wirings on the semiconductor element, resulting in failures in the semiconductor device and hence a reduction in high temperature reliability.
In order to solve these problems, an approach has been proposed which comprises the use of a non-halogen and non-antimony metal hydroxide as an inorganic flame retardant. However, this approach gives a rise to another problem.
The first problem is attributable to swelling caused by dehydration reaction at the postcuring step. A surface mounting has been mainly used as the method for mounting a semiconductor package. Thus, solder dipping process, infrared reflow process, vapor phase reflow process, etc. are employed. In these processes, the package is exposed to high temperatures (normally 215.degree. to 260.degree. C.). Therefore, in a semiconductor device encapsulated with a resin composition comprising a large amount of a metal hydroxide incorporated therein, water content produced by dehydration reaction and a minute amount of water content which has penetrated into the semiconductor device through the encapsulating resin are suddenly vaporized, resulting in the reduction of the reliability guarantee for humidity resistance. Further, the encapsulating resin swells, making it impossible to package the semiconductor element.
Referring to the second problem, the high temperature reliability guarantees the function of semiconductors in an atmosphere of as high as 150.degree. to 200.degree. C. Semiconductor elements which generate a large amount of heat or semiconductor devices adapted to be mounted in the vicinity of automobile engine are disadvantageous in that they are susceptible to reduction of high temperature reliability due to dehydration reaction after prolonged use.
As described above, the conventional flame-retarding technique gives rise to the above-described problems. It has thus been keenly desired to develop a flame-retarding technique which is safe and unharmful to human body and environment and insusceptible to the reduction of the crack resistance due to the dehydration of metal hydroxide at infrared reflow step and corrosion of aluminum wirings on the semiconductor element or reduction of high temperature reliability after prolonged storage in an atmosphere of a high temperature and humidity.
In the light of such circumstances, the present invention has been worked out. An object of the present invention is to provide a semiconductor device which exhibits a high reliability at high temperatures and high heat resistance, flame retardance and safety at infrared reflow step.